Preheat oven to 350°F (175°C). Line an 8x8 inch metal baking pan with parchment paper, leaving overhangs. Grease parchment.

In a large microwave-safe bowl, combine butter and chopped semisweet chocolate. Microwave in 30-second bursts, stirring, until melted and smooth. Let cool 5 minutes.

Whisk both granulated and brown sugars into the melted chocolate mixture until combined. Whisk in eggs, one at a time, followed by vanilla extract.

Sift cocoa powder directly into the bowl. Add flour and salt. Genty fold the dry ingredients in with a spatula until just combined. Mix only until the flour streaks disappear.

Warm the cookie butter in the microwave for 15-20 seconds until molten. Pour half of the brownie batter into the pan and spread evenly.

Drizzle half of the warm cookie butter over the batter. Add the remaining brownie batter and spread. Drizzle the rest of the cookie butter on top.

Use a knife to gently swirl the cookie butter through the batter. Sprinkle with flaky sea salt if using.

Bake for 30-35 minutes, until the top is set but the center is still soft. A toothpick inserted should come out with moist crumbs.

Let cool completely in the pan on a wire rack before removing and slicing. For maximum molten texture, serve slightly warm.